The present disclosure relates to a fixing device to fix a toner image on a sheet and an image forming apparatus including the fixing device.
In the fixing device, between a fixing belt heated by a heater and a pressing member, a pressing area is formed. When a sheet on which a toner image is transferred is passed through the pressing area, the toner image is heated and pressed to be fixed on the sheet. Recently, in consideration of the fact that high heat efficiency can be obtained, a fixing device of a type in which a heater having a board and a heating element arranged on the board is made to come into contact with an inner circumferential face of the fixing belt has been proposed.
Because such a heater requires large current, if contact resistance between a power supplying electrode and a power supplying connector is large, heat is generated at a contact portion between the electrode and the connector. Thereby, it is required to make contact pressure between them large and to lower the contact resistance. This makes the contact portion large in size.
Additionally, because large electrical resistance of conduction paths connecting the power supplying electrode to the heating element may cause heat generation, it is difficult to arrange the conduction paths without crossing with the contact portion. If the contact portion between the electrode and the connector is overlapped with the conduction paths, when the connecter is connected to the electrode with strong force, an insulator covering a surface of the conduction paths may be scraped by the connector. Accordingly, it is difficult to reduce a number of the connector without crossing the conduction paths.
Then, in some cases, a heating area is divided for a case where a sheet passed through the pressing area has a wide width and a case where a sheet passed through the pressing area has a narrow width. An example of such a heater will be described with reference to FIG. 6 and FIG. 7. FIGS. 6 and 7 are plan views showing the heater. In an example shown in FIG. 6, the heating area of the heater 100 is divided into a center heating part 101 arranged on a center side in the sheet width direction X and end heating parts 103 arranged on both outer side of the center heating part 101. The center heating part 101 corresponds to the sheet having a narrow width while the center heating part 101 and the end heating parts 103 correspond to the sheet having a wide width. On both end faces in the width direction X of each of the center heating part 101 and the end heating parts 103, electrodes 105 and 107 are provided. The heater 100 includes a common electrode 109, a first individual electrode 111 and a second individual electrode 113. The common electrode 109 is arranged at one outer side of the end heating parts 103 in the width direction X, and the first and second individual electrodes 111 and 113 are arranged at the other outer side of the end heating parts 103 in the width direction X.
The center heating part 101 is connected between the common electrode 109 and the first individual electrode 111. The end heating parts 103 are connected between the common electrode 109 and the second individual electrode 113 in series. In the example, in order to make a heating value per unit length (length along the width direction X) of each of the center heating part 101 and the end heating parts 103 same, the heating element is changed in its thickness and its length along a conveyance direction Y perpendicular to the width direction X and to adjust a resistance per unit length in the width direction X. This makes it difficult to form the heater.
On the other hand, as shown in FIG. 7, in a case where the electrodes 105 and 107 are provided on both end faces in the conveyance direction Y of each of the center heating part 101 and the end heating parts 103, if the center heating part 101 is divided into two parts in the width direction X, the thickness and the length in the conveyance direction Y of the heating parts becomes uniform. In this case, if the resistance per unit length in the width direction X of the heating parts are made to be uniform, it becomes possible for the divided two center heating parts 101 and the end heating parts 103 to have the same heating value per unit length (the length along the conveyance direction).
However, between the electrodes of the divided two center heating parts 101 (between the electrodes which is not connected to the conduction path, a portion surrounded by a circle in FIG. 7), a voltage applied between the common electrode 109 and the first individual electrode 111 is applied. Thereby, it is required to give a space between the divided two heating parts 101 in order to prevent a short circuit. Then, a temperature distribution in the width direction X is uneven around the space, and a fixing performance may be affected.